On the Males and Complemental Males of certain
Cirripedes, and on Rudimentary Structures.

I BEG permission to make a few remarks bearing on Prof. Wyville
Thomson's interesting account of the rudimentary males of Scalpellum
regium, in your number of August 28th.1 Since I described in 1851,2 the males and complemental males of certain cirripedes, I have been
most anxious that some competent naturalist should re-examine them;
more especially as a German,3 without apparently having taken the
trouble to look at any specimens, has spoken of my description as a
fantastic dream. That the males of an animal should be attached to the
female, should be very much smaller than, and differ greatly in
structure from her, is nothing new or strange. Nevertheless, the
difference between the males and the hermaphrodites of Scalpellum
vulgare is so great, that when I first roughly dissected the
former, even the suspicion that they belonged to the class of
cirripedes did not cross my mind. These males are half as large as the
head of a small pin; whereas the hermaphrodites are from an inch to an
inch and a quarter in length. They consist of little more than a mere
sack, containing the male reproductive organs, with rudiments of only
four of the valves; there is no mouth or alimentary canal, but there
exists a rudimentary thorax with rudimentary cirri, and these
apparently serve to protect the orifice of the sack from the intrusion
of enemies. The males of Alcippe and Cryptophialus are even more
rudimentary; of the seventeen segments which ought to be fully
developed, together with their appendages, only three remain, and these
are imperfectly developed; the other fourteen segments are represented
by a mere slight projection bearing the probosci-formed penis. This
latter organ, on the other hand, is so enormously developed in
Cryptophialus, that when fully extended it must have been between eight
and nine times the length of the animal! There is another curious point
about these little males, viz., the great difference between those
belonging to the several species of the same genus Scalpellum: some are
manifestly pedunculated cirripedes, differing by characters which in an
independent creature would be considered as of only generic value;
whereas others do not offer a single character by which they can be
recognised as cirripedes, with the exception of the cast-off
prehensile, larval antennæ, preserved by being buried in the natural
cement at the point of attachment. But the fact which has interested me
most is the existence of what I have called Complemental Males, from
their being attached not to females, but to hermaphrodites; the latter
having male organs perfect, although not so largely developed as in
ordinary cirripedes. We must turn to the vegetable kingdom for anything
analogous to this; for, as is well known, certain plants present
hermaphrodite and male individuals, the latter aiding in the
cross-fertilisation of the former. The males and complemental males in
some of the species of three out of the four very distinct genera in
which I have described their occurrence, are, as already stated,
extremely minute, and, as they cannot feed, are short-lived. They are
developed like other cirripedes, from larvæ, furnished with
well-developed natatory legs, eyes of great size and complex prehensile
antennæ; by these organs they are enabled to find, cling to, and
ultimately to become cemented to the hermaphrodite or female. The male
larvæ, after casting their skins and being as fully developed as they
ever will be, perform their masculine function, and then perish. At the
next breeding season they are succeeded by a fresh crop of these annual
males. In Scalpellum vulgare I have found as many as ten
males attached to the orifice of the sack of a single hermaphrodite;
and in Alcippe, fourteen males attached to a single female.

He who admits the principle of evolution will naturally inquire why
and how these minute rudimentary males, and especially the complemental
males, have been developed. It is of course impossible to give any
definite answer, but a few remarks may be hazarded on this subject. In
my "Variation under Domestication," I have given reasons for the belief
that it is an extremely general, though apparently not quite universal
law, that organisms occasionally intercross, and that great benefit is
derived therefrom. I have been laboriously experimenting on this
subject for the last six or seven years, and I may add, that with
plants there cannot be the least doubt that great vigour is thus
gained; and the results indicate that the good depends on the crossed
individuals having been exposed to slightly different conditions of
life. Now as cirripedes are always attached to some object, and as they
are commonly hermaphrodites, their intercrossing appears, at first
sight, impossible, except by the chance carriage of the spermatic fluid
by the currents of the sea, like pollen by the wind; but it is not
probable that this can often happen, as the act of impregnationt takes
place within the well-enclosed sack. As, however, these animals possess
a probosci-formed penis capable of great elongation, two closely
attached hermaphrodites could reciprocally fertilise each other This,
as I have elsewhere proved, does sometimes, perhaps often, actually
occur. Hence perhaps it arises, that most cirripedes are attached in
clusters. The curious Anelasma, which lives buried in the skin of
sharks in the northern seas, is said always to live in pairs. Whilst
reflecting how far cirripedes

usually adhered to their support in clusters, the case of the genus
Acasta occurred to me, in which all the species are embedded in
sponges, generally at some little distance from each other; I then
turned to my description of the animal, and found it stated, that in
several of the species the probosci-formed penis is "remarkably long;"1
and this I think can hardly be an accidental coincidence. With respect
to the habits of the genera which are provided with true males or
complemental males:—all the species of Scalpellum, excepting one, are
specially modified for attachment to the delicate branches of
corallines: the one species of Ibla, about which I know anything, lives
attached, generally two or three together, to the peduncle of another
cirripede, viz. a Pollicipes: Alcippe and Cryptophialus are embedded in
small cavities which they excavate in shells. No doubt in all these
cases two or more full-grown individuals might become attached close
together to the same support; and this sometimes occurs with Scalpellum
vulgare, but the individuals in such groups are apt to be
distorted and to have their peduncles twisted. There would be much
difficulty in two or more individuals of Alcippe and Cryptophialus
living embedded in the same cavity. Moreover, it might well happen that
sufficient food would not be brought by the currents of the sea to
several individuals of these species living close together.
Nevertheless in all these cases it would be a manifest advantage to the
species, if two individuals could live and flourish close together, so
as occasionally to intercross. Now if certain individuals were reduced
in size and transmitted this character, they could readily be attached
to the other and larger individuals; and as the process of reduction
was continued, the smaller individuals would be enabled to adhere
closer and closer to the orifice of the sack, or, as actually occurs
with some species of Scalpellum and with Ibla, within the sack of the
larger individual; and thus the act of fertilisation would be safely
effected. It is generally admitted that a division of physiological
labour is an advantage to all organisms; accordingly, a separation of
the sexes would be so to cirripedes, that is if this could be effected
with full security for the propagation of the species. How in any case
a tendency to a separation of the sexes first arises, we do not know;
but we can plainly see that if it occurred in the present case, the
smaller individuals would almost necessarily become males, as there
would be much less expenditure of organic matter in the production of
the spermatic fluid than of ova. Indeed with Scalpellum vulgare the
whole body of the male is smaller than a single one of the many ova
produced by the hermaphrodite. The other and larger individuals would
on the same principle either remain hermaphrodites, but with their
masculine organs more or less reduced, or would be converted into
females. At any rate, whether these views are correct or not, we see at
the present time within the genus Scalpellum a graduated series: first
on the masculine side, from an animal which is obviously a pedunculated
cirripede with well-proportioned valves, to a mere sack enclosing the
male organs, either with the merest rudiments of valves, or entirely
destitute of them; and secondly on the feminine side, we have either
true females, or hermaphrodites with the male organs perfect, yet
greatly reduced.

With respect to the means by which so many of the most important
organs in numerous animals and plants have been greatly reduced in size
and rendered rudimentary, or have been quite obliterated, we may
attribute much to the inherited effects of the disuse of parts. But
this would not apply to certain parts, for instance to the calcareous
valves of male cirripedes which cannot be said to be actively used.
Before I read Mr. Mivart's2 acute criticisms on this subject, I thought
that the principle of the economy of growth would account for the
continued reduction and final obliteration of parts; and I still think,
that during the earlier periods of reduction the process would be thus
greatly aided. But if we consider, for instance, the rudimentary
pistils or stamens of many plants, it seems incredible that the
reduction and final obliteration of a minute papilla, formed of mere
cellular tissue, could be of any service to the species. The following
conjectural remarks are made solely in the hope of calling the
attention of naturalists to this subject. It is known from the
researches of Quetelet3 on the height of man, that the number of
individuals who exceed the average height by a given quantity is the
same as the number of those who are shorter than the average by the
same quantity; so that men may be grouped symmetrically about the
average with reference to their height. I may add, to make this
clearer, that there exists the same number of men between three and
four inches above the average height, as there are below it. So it is
with the circumference of their chests; and we may presume that this is
the usual law of variation in all the parts of every species under
ordinary conditions of life. That almost every part of the body is
capable of independent variation we have good reason to believe, for it
is this which gives rise to the individual differences characteristic
of all species. Now it does not seem improbable that with a species
under unfavourable conditions, when, during many generations, or in
certain areas, it is pressed for food and exists in scanty numbers,
that all or most of its parts should tend to vary in a greater number
of individuals towards diminution than towards increment of size; so
that the grouping would be no longer symmetrical with reference to the
average size of any organ under consideration. In this case the
individuals which were born with parts diminished in size and
efficiency, on which the welfare of the species depended, would be
eliminated; those individuals alone surviving in the long run which
possessed such parts of the proper size. But the survival of none would
be affected by the greater or less diminution of parts already reduced
in size and functionally useless. We have assumed that under the above
stated unfavourable conditions a larger number of individuals are born
with any particular part or organ diminished in size, than are born
with it increased to the same relative degree; and as these
individuals, having their already reduced and useless parts still more
diminished by variation under poor conditions, would not be eliminated,
they would intercross with the many individuals having the part of
nearly average size, and with the few having it of increased size. The
result of such intercrossing would be, in the course of time, the
steady diminution and ultimate disappearance of all such useless parts.
No doubt the process would take place with excessive slowness; but this
result agrees perfectly with what we see in nature; for the number of
forms possessing the merest traces of various organs is immense. I
repeat that I have ventured to make these hypothetical remarks solely
for the sake of calling attention to this subject.